Low threshold stripe geometry injection laser

ABSTRACT

The threshold current density is reduced in a stripe geometry injection laser which includes at least one additional control stripe contact parallel to and spaced from the stripe contact normally utilized to produce lasing. The current applied to the control stripe is maintained below that required for lasing, thereby reducing the optical loss in the region under the control stripe. Since the optical field normally penetrates into this region, the total loss of the laser as a whole is reduced resulting in a lower threshold for lasing. Embodiments utilizing a control stripe on both sides of the active stripe are also described.

United States Patent Miller [54] LOW THRESHOLD STRIPE GEOMETRY INJECTIONLASER [72] Inventor: Stewart Edward Miller, Middletown T heMenmesthQEELNL [73] Assignee: Bell Telephone Laboratories,Incorporated, Murray Hill, Berkeley Heights, NJ.

22 Filed: Dec.9,1970

21 Appl. No.: 96,530

[52] US. Cl. ..331/94.5 H, 317/234 R [51] Int. Cl H018 3/00 [58] Fieldof Search ..33l/94.5; 317/235 Nathan et al: GaAs Injection Laser withNovel Mode Control and Switching Properties, J. of App. Phys,

[ NOV. 14, 1972 Vol. 36, PP. 473- 480, Feb., 1965.

Dyment et a1: Continuous Operation of GaAs Junction Lasers on DiamondHeat Sinks at 200 K, App. Phys. Litt, Vol. 11, pp. 292- 294, Nov. 1967.

Primary Examiner-Ronald L. Wibert Assistant Examiner-Edward S. BauerAttorneyR. J. Guenther and Arthur J. Torsiglieri [5 7] ABSTRACT Thethreshold current densityis reduced in a stripe geometry injection laserwhich includes at least one additional control stripe contact. parallelto and spaced from the stripe contact normally utilized to producelasing. The current applied to the control stripe is maintained belowthat required for lasing, thereby reducing the optical loss in theregion under the control stripe. Since the optical field normallypenetrates into this region, the total loss of the laser as a whole isreduced resulting in a lower threshold for lasing. Embodiments utilizinga control stripe on both sides of the active stripe are also described.

1 Claim, 3 Drawing Figures P'A'TENTED um 14 1972 FIG.

FIG. 2

FIG. 3

,.-- -T|N PLATED TIN-PLATED COPPER 52 INVENTOR 5. E. MILLER B) ATTORNEYLOW THRESHOLD STRIPE GEOMETRY INJECTION LASER BACKGROUND OF THEINVENTION This invention relates to semiconductor lasers and,

more particularly, to Semiconductor stripe geometry injection lasershaving reduced thresholds for lasing.

Since the advent of the semiconductor laser, workers skilled in the arthave long tried to achieve continuous wave (c.w.) operation at roomtemperature. One difficulty has been that high optical losses coupledwith limited gain results in prohibitively high current densitythresholds for lasing at room temperature, typically about 40,000 AlcmThe basic problem, therefore, is the inability to remove rapidly enoughthe heat generated in the small volume of the junction region before thetemperature rises to such a point that the semiconductor is damaged. Toalleviate this problem, and still attain c.w. operation, workers untilrecently have resorted to operation at cryogenic temperatures where thethresholds are considerably less, typically about 1,000 A/cm and hencethe temperature rise is not nearly so severe. More recently, I. Hayashiet al have reported in Applied Physics Letters, 17, 109 (1970)successful c.w. operation at room temperature in a doubleheterostructure injection laser. This laser, which forms a part of thesubject matter of copending application Ser. No. 33,705 filed May 1,1970 (I. Hayashi Case 4 comprises a narrow band gap active regiondisposed between opposite conductivity type wider band gap regions. Thedouble heterostructure laser is characterized not only by lowerthresholds at room temperature (e.g., 1,000 A/cm but also by a lowertemperature dependence of threshold, both of which result primarily fromthe effect of carrier confinement in the narrow active region.

In both the conventional laser diode and the improved doubleheterostructure, it is desirable to utilize a stripe electrical contactto the p-region in order to obtain the well known advantages oftransverse mode control and improved thermal properties. While such astripe contact tends to confine the lasing to the low loss junctionregion under the contact, tail portions of the lowest order mode(Gaussian) optical field extend beyond that region into immediatelyadjacent high loss regions. Since these tail portions experience highloss in the adjacent regions, the effective threshold for lasing isgreater than it would be if the adjacent regions were low optical lossregions.

It is therefore an object of my invention to reduce the optical loss inthe junction regions of stripe contact laser diode adjacent the lasingstripe contact.

It is another object of my invention to reduce the lasin g threshold ofjunction laser diodes.

SUMMARY OF THE INVENTION These and other objects are accomplished in anillustrative embodiment of my invention, a stripe contact geometrysemiconductor injection laser in which at least one control stripecontact is disposed parallel to and spaced from the normal active lasingcontact. Current is maintained above threshold in the lasing contactwhereas it is maintained below threshold in the control stripe with theresult that the optical loss in the regions adjacent the lasing contactis reduced and hence the threshold is also reduced.

The manner in which the optical loss is reduced is briefly explained asfollows. In conventional diodes little of the pumping current flows inthe regions adjacent the lasing contact,and hence in those regions nopopulation inversion exists between the valence and conduction bands.Consequently, the tails of the optical field which extend therein exciteelectrons from the valence to the conduction band by absorption, a formof optical loss. The amount of absorption is directly proportional tothe number of electrons so excited. When, however, the laser is providedwith control stripe contacts in accordance with my invention, a largeportion of the valence band electrons in the adjacent regions areexcited to the conduction band by the control current flowing in theseregions; hence, fewer electrons remain in the valence band which. cancause optical absorption. It should be noted that the current in thecontrol stripes must be maintained below threshold in order to attainthese advantages. Otherwise, the diode would operate as a single widecontact laser instead of a narrow stripe contact laser and therebyforfeit the well known advantages of stripe contact lasers related tomode control and thermal properties.

In fact, it is preferable to utilize in my invention a pair of parallelcontrol stripes, one on either side of the lasing stripe, in order toreduce the optical loss in the regions of penetration of both tails ofthe Gaussian optical field distribution.

BRIEF DESCRIPTION OF THE DRAWING These and other objects of theinvention, together with its various features and advantages, can bemore easily understood from the following more detailed descriptiontaken in conjunction with the accompanying drawing, in which:

FIG. 1 is a top view of a laser diode in accordance with an illustrativeembodiment of my invention;

FIG. 2 is a side of the diode of FIG. 1 including illustrative circuitconnections; and

FIG. 3 is a side view of the diode of FIG. I mounted on an illustrativeheat sink.

DETAILED DESCRIPTION Turning now to FIGS. 1 and 2, there is shown apreferred embodiment of my invention, a semiconductor laser diode 10comprising contiguous p and n regions 12 and 14 forming a pm junction 16therebetween. Onto the bottom of the n-region 14 is deposited a metalliccontact 18, whereas on the top of p-region 12 are deposited a lasing oractive stripe electrical contact 20 and a pair of parallel controlstripe electrical contacts 22 and 24 disposed on either side of lasingcontact 20. End faces 26 and 28 are typically cleaved or polishedoptically flat and perpendicular to junction 16 to form an opticalcavity resonator for sustaining coherent radiation generated in thejunction. One of the end faces is made nearly totally reflective whereasthe other is made partially transmissive in order to provide a means ofegress for the coherent radiation.

As shown in FIG. 2, a source 30 is connected between lasing stripe 20and contact 18 in order to supply forward bias current I, in excess ofthe lasing threshold I In addition, sources 32 and 34 are connectedbetween contact 18 and control stripes 22 and 24, respectively, in orderto supply forward bias currents I and 1 respectively, each of which areless than I, (assuming, for the moment, contacts of the same length andwidth).

For the purposes of further discussion, it will be assumed that thelaser parameters including the pump current I and the width of lasingstripe 20 are chosen so that the laser 10 oscillates in its fundamentaltransverse mode only. Thus, the optical field in the junction region inthe dimension transverse to the resonator axis is Gaussian as shown bycurve 36 of FIG. 2. It should be noted that the Gaussian curve 36 isdrawn in the p-region 12 for convenience only, it being understood thatin fact the optical field is essentially limited to the junction regiononly.

Due to current spreading under contact 20, diffraction and relatedeffects, the optical field is not confined to the junction region 38under contact 20, but penetrates into adjacent high loss regions 40 and42. As discussed previously, regions 40 and 42 are regions of highoptical absorption and hence the losses incurred therein contributesignificantly to the threshold level of the laser.

In order, therefore, to reduce the threshold, I propose that controlstripes 22 and 24 be positioned adjacent lasing stripe 20 and directlyabove the junction regions 40 and 42 into which the tails of the opticalfield penetrate. By maintaining the currents in the control stripes lessthan threshold; i.e., less than that required to give net gain in theregions 40 and 42, but sufficient to produce nearly zero loss in theseregions, the optical loss of the diode as a whole is reduced andtherefore so is the threshold.

Lower thresholds naturally imply less heating of the junction which isespecially advantageous for c.w. operation of laser diodes at roomtemperature. In one such laser diode 10, as shown in FIG. 3, contact 18is bonded to a metallized (e.g., tin-plated) high thermal conductivitydiamond 50 mounted on a tin-plated copper heat sink 52. Due to thedifficulty in getting the tin plating to cover the entire diamond 50,gold wires 56 and 58 (about 25 pm in diameter) may be used to ground thetin-plated top surface 54 of the diamond to the heat sink 52.

Illustratively, the laser diode 10 is a double heterostructure junctionlaser which operates c.w. at room temperature as described in theaforementioned application, I. I-Iayashi Case 4. The diode comprisestypically a thin layer of p-type GaAs (about 1 pm thick or less)sandwiched between pand n-type layers of wider band gap Ga Al As,respectively, about 1.1 um and pm thick. Such diodes may be fabricatedby a liquid phase epitaxial technique as described by M. B. Panish andS. Sumski in copending application Ser. No. 28,365 filed Apr. 14, I970(Case 55 In this regard, see also Applied Physics Letters, 17, 109(1970). Before depositing the metal to form the stripe contacts, whichmay be defined by standard photolithographic techniques, a dopant suchas Zn is typically diffused into the p-Ga, ,Al,,As to form a shallow(e.g., 0.2 am) p layer to provide ohmic contact. Subsequently, the diodeis bonded to a heat sink as previously described with reference to FIG.3.

Diodes fabricated by this procedure operate continuously at roomtemperature (e.g., A 8,858 A.) with c.w. thresholds of about 0.3 A(6,000 A/cm) for a lasing stripe 13 um wide by 400 um long, the diodewidth being about um and its depth, including an n-type 'GaAs substrate,being about 6-7 mils. In accordance with my invention, therefore,control stripes illustratively 3-10 um wide and 400 um long spaced fromthe lasing stripes by a distance of 2-5 pm are added to the singlestripe diode. To reduce the threshold below 6,000 A/cm the current inboth stripes should be maintained such that current density under thecontrol stripes is less than 6,000 Alcm Thus if the control stripes havethe same area as the lasing stripe, the current in the control stripesshould be less than 0.3 A for this example. If, however, the controlstripes are narrower (e.g. 6 pm) than the lasing stripe, then thecontrol current should be correspondingly less (e.g. less than 0.15A).

It is to be understood that the above-described arrangements are merelyillustrative of the many possible specific embodiments which can bedevised to represent application of the principles of the invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention. In particular, my invention alsocontemplates the use of a plurality of active lasing stripe contactsinterleaved with control stripes to reduce. the optical loss betweenlasing stripes.

In an appropriate semiconductor medium it may also be possible to reducethe optical loss in the regions adjacent the lasing stripe by meansother than the aforementioned parallel control stripes, e. g., bywidening the band gap in those regions by ion implantation, or diffusionof impurities.

In addition, diodes fabricated in accordance with my invention may alsoexhibit self-induced pulsing at microwave rates as described by J. E.Ripper and T. L. Paoli in an article entitled Coupled Longitudinal ModePulsing in Semiconductor Lasers, Physics Review Letters, 22, 1,085 (May26, 1969).

What is claimed is:

1. A semiconductor injection laser comprising a semiconductor activemedium having a planar p-n junction therein for generating coherentradiation in the plane of said junction and "further having first andsecond major surfaces parallel to said plane,

means forming an optical resonator having an optic axis parallel to saidjunction, and including therein said medium, for sustaining saidradiation,

a lasing stripe electrical contact and a pair of control stripeelectrical contacts, one of said control contacts being located oneither side of said lasing contact and substantially parallel thereto,said lasing and control contacts being electrically isolated from oneanother and being located on said first major surface with the elongateddimension of said contacts extending parallel to the optic axis of saidresonator,

said lasing contact being about 13 um wide, said control contacts beingabout 10 um wide and the separation between said lasing contact and eachof said control contacts being about 5 pm,

a third electrical contact located on said second major surface,

a heat sink bonded to said third contact to remove heat generated insaid medium,

means for reducing the optical loss in first regions of said junctionunder each of said control contacts comprising means for applyingbetween said third contact and said control contacts forward biascurrent so that the current density in said first regions is less thanthat required to produce net gain in said first regions, and

1. A semiconductor injection laser comprising a semiconductor activemedium having a planar p-n junction therein for generating coherentradiation in the plane of said junction and further having first andsecond major surfaces parallel to said plane, means forming an opticalresonator having an optic axis parallel to said junction, and includingtherein said medium, for sustaining said radiation, a lasing stripeelectrical contact and a pair of control stripe electrical contacts, oneof said control contacts being located on either side of said lasingcontact and substantially parallel thereto, said lasing and controlcontacts being electrically isolated from one another and being locatedon said first major surface with the elongated dimension of saidcontacts extending parallel to the optic axis of said resonator, saidlasing contact being about 13 Mu m wide, said control contacts beingabout 10 Mu m wide and the separation between said lasing contact andeach of said control contacts being about 5 Mu m, a third electricalcontact located on said second major surface, a heat sink bonded to saidthird contact to remove heat generated in said medium, means forreducing the optical loss in first regions of said junction under eachof said control contacts comprising means for applying between saidthird contact and said control contacts forward bias current so that thecurrent density in said first regions is less than that required toproduce net gain in said first regions, and means for generating saidcoherent radiation comprising means for applying between said thirdcontact and said lasing contact forward bias current so that the currentdensity in a second region of said junction under said lasing contactexceeds that required for net gain when said first regions areforwardbiased at a current level less than that required to produce netgain in said first regions.